1. Field of Invention
The invention is directed to a structural joint for and a method of joining a reinforced thermoplastic element to another element, and, more particularly, a method of fabricating an aerodynamic structure including a reinforced thermoplastic component.
2. Description of Related Art
Carbon fiber reinforced materials are widely used as structural components, particularly in the aircraft industry. Currently, construction of wings for aircraft incorporate carbon fiber reinforced thermoplastic support structures including spars to which is secured a metal skin. Traditional rivets are used to join the skin to the spars requiring attachment to the thermoplastic spars of a metal flange for receiving the rivet.
While the use of thermoplastic support structures reduces weight without decreasing strength, the need for the metal flange for receiving rivets introduces additional unnecessary manufacturing steps and does not eliminate the traditional corrosion problems encountered at the point of riveting. Moreover, because of the different coefficients of thermal expansion of the metal flange and the thermoplastic spar, differential strains are introduced which were not present in previous all metal wings.
Efforts have been made to overcome the disadvantages of metal rivets with thermoplastic support structures by adhesively securing the skin to the structure. The adhesives, however, have less strength than is generally required.
Fiber reinforced thermoplastic rivets have been taught by Berecz in U.S. Pat. Nos. 4,687,394, 4,687,395, 4,687,397, and 4,687,398. Other plastic rivets are shown in Strand U.S. Pat. No. 4,478,544 and Lyon U.S. Pat. No. 4,478,543. All of these patents are directed to separate, preformed, cylindrical, thermoplastic rivets requiring individual insertion into coaxial bores and requiring complex structural apparatus for securing each individual rivet into position. Moreover, the rivets taught in these patents generally are not usable in modern aircraft construction because they require projecting rivet heads which are aerodynamically unacceptable.
German Patent No. 8,578,868 discloses a joint and method of forming the joint between a non-fiber reinforced plastic element and a metal element in which a tab extending from the plastic element is inserted into a truncated conical bore and heat deformed to fill the bore and secure the two elements together. The patent apparently is directed to attachment of thermoplastic electric condensor housings to metal support plates. Such a device is not subjected to the extreme forces encountered in aircraft.
All of the patents addressed above teach cylindrical rivet constructions. Cylindrical attachments do not permit selective accommodation of asymmetrical forces encountered at the attachment point. Depending upon its location and use, each attachment point in an aircraft is subjected to different shear, tension and bending forces applied in different directions. A cylindrical attachment structure is generally symmetrical and must be designed to resist the highest expected force, particularly in transverse tension and bending. This may entail an attachment structure having greater strength than necessary in one direction to resist expected forces in another direction, or use of additional attachment structures to accommodate expected forces in one direction resulting additional cost and complexity, or, in the case of design compromises, an attachment structure having insufficient strength in one direction.
The present invention is directed to a structural joint for connecting a fiber reinforced thermoplastic element to another element. The invention eliminates the need for traditional metal rivets and the associated metal flange on the thermoplastic element thereby reducing weight. The invention eliminates the need for separate attaching components such as taught in the Berecz patents. The invention contributes to efficient manufacture by permitting automated assembly of the components. The invention provides the high strength connection required for aerodynamic structures while permitting asymmetrical strength characteristics. The invention reduces or eliminates the corrosion traditionally encountered in metal fastening joints. The invention reduces the effect of differential thermal expansion. Overall, the invention significantly improves the ability to efficiently manufacture high strength structures including a fiber reinforced thermoplastic component.
Additional advantages of the invention are set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.